Optical-fiber Bragg gratings (FBGs) have become essential components in the telecommunications industry, where they perform various spectral filtering operations. A fiber Bragg grating consists of a periodic modulation of the index of refraction along the core of an optical fiber. It is created by exposing a photosensitive fiber to a properly shaped intensity pattern of ultraviolet light. This light produces a permanent change in the index of refraction in selected sections of the optical fiber. The resulting optical fiber grating behaves as a wavelength-selective reflector. The reflected wavelength of light is often referred to as the grating wavelength or as the Bragg wavelength of the grating. Their stability and reliability, in conjunction with their all-guided-wave nature has made FBGs ideal candidates for fiber optic system applications.
Fiber Bragg gratings are now used extensively in the field of optical telecommunications, e.g. for Wavelength Division Multiplexing (WDM), for compensating chromatic dispersion in optical fibers, for stabilizing and flattening the gain of optical amplifiers, for stabilizing the frequency of semiconductor lasers, and more generally in various filters. They are also found in instrumentation, e.g. as narrow band wavelength-selective reflectors for fiber lasers, or as sensors for the measurement of strain, temperature, and hydrostatic pressure.
The carrying capacity of WDM systems can be increased by using more and more narrowly spaced channels within an overall transmission spectral band. This requires the use of filters with an accurately defined spectral response. The Bragg wavelength of a grating depends on the period of modulation and on the average value of the refractive index in the fiber. Both quantities can be varied by straining the optical fiber. Fine-tuning of the spectral response of a fiber Bragg grating can thus be achieved by straining the optical fiber containing said grating in a well controlled manner.
Current telecommunication applications require the Bragg wavelength of FBGs to be accurate within a few tens of picometers. This requires a submicron-level control of the length of the gratings, achievable only with a fine-tuning mechanism. Several systems have been proposed in the past to achieve this goal of precisely adjusting the wavelength of optical devices, especially in the context of athermal packaging of fiber devices.
Referring for example to U.S. Pat. No. 5,042,898 (MOREY et al.) and G. W. Yoffe, P. A. Krug, F. Ouellette, and D. A. Thorncraft, “Passive temperature-compensating package for optical fiber gratings”, Appl. Optics, Vol. 34, No. 30, October 1995, pp. 6859-6861 the use of screws extending along the longitudinal axis of an optical fiber has been suggested to apply tension to a grating in this fiber. However, the accuracy of this method is limited by the number of threads per unit length on the screw. Typical implementations did not provide a sufficiently accurate control of the Bragg wavelength. To improve this accuracy, assignee's co-pending U.S. application Ser. No. 09/952.715, filed on Sep. 12, 2001, suggest the addition of a locking nut to the structure to improve the control of the Bragg wavelength. The sequential rotation of the tension screw and locking nut with respect to one another provides a better control of the axial displacement than that achievable with only the tension screw.
More elaborate methods have also been presented to improve accuracy, like differential threaded elements (U.S. Pat. No. 5,991,483 to ENGELBERTH), tweaker screws (see U.S. Pat. No. 6,101,301 to ENGELBERTH et al.) or elaborate pivoting lever systems (for example in U.S. Pat. No. 6,147,341 (LEMAIRE et al.) and U.S. Pat. No. 6,144,789 (ENGELBERTH et al.)). These systems have not proven much successful, because of implementation difficulties or a lack of mechanical stability.